The initial impetus for changing to AJAX for Web applications is to improve the user experience by avoiding screen refreshes and increasing the speed of interaction. Another benefit, however, is that others can take the services your application is built on and build new ones. I reported earlier on an experiment by Mike Teets with Google maps. His first prototype broke after two days because Google changed something in their implementation. This is the sort of thing that an implementation based on standard protocols would help avoid.